Cation groups I–V — group reagents, separation scheme

3.5.1 · Chemistry › Inorganic Qualitative Analysis

WHY hum separate karte hain aakhir?

YEH humein kya deta hai: $[B^{n -}]$ (group reagent ke anion) ko control karke, hum control karte hain ki kaun se cations ke liye $Q > K_{s p}$ hoga. Yahi poore scheme ki logic hai.

Paanch analytical groups

Definition Group, group reagent, basis

Group	Cations	Group reagent (conditions)	Precipitate type	Basis
I	$Pb^{2 +}, Ag^{+}, Hg_{2}^{2 +}$	dilute HCl	chlorides	chhote $K_{s p}$ wale chlorides
II	$Cu^{2 +}, Cd^{2 +}, Bi^{3 +}, Hg^{2 +}, Pb^{2 +}, As^{3 +}, Sb^{3 +}, Sn^{2 +}$	== $H_{2} S$ in acidic== medium (dil. HCl)	sulphides	low $K_{s p}$ sulphides
III	$Fe^{3 +}, Al^{3 +}, Cr^{3 +}$	== $N H_{4} C l + N H_{4} O H$ ==	hydroxides	low $K_{s p}$ hydroxides
IV	$Co^{2 +}, Ni^{2 +}, Mn^{2 +}, Zn^{2 +}$	== $H_{2} S$ in basic== medium ( $N H_{4} O H$ )	sulphides	thoda zyada $K_{s p}$ wale sulphides
V	$Ba^{2 +}, Sr^{2 +}, Ca^{2 +}$	== $(N H_{4})_{2} C O_{3}$ == (ammoniacal)	carbonates	sirf carbonates insoluble hain

(Group VI / "zero": $Mg^{2 +}, Na^{+}, K^{+}, NH_{4}^{+}$ — solution mein hi rehte hain, flame/special tests se milte hain.)

Figure — Cation groups I–V — group reagents, separation scheme

H₂S ka split (Group II vs IV) actually kaise kaam karta hai — derive karte hain

Yeh sabse clever part hai, toh hum ise scratch se banate hain.

Step 1 — H₂S do genuine steps mein dissociate karta hai. $H_{2} S$ ek weak diprotic acid hai, aur dono steps ki strength bahut alag hai: $H_{2} S ⇌ H^{+} + H S^{-}, K_{a 1} = \frac{[ H ^{+} ] [ H S ^{-} ]}{[ H _{2} S ]} (\sim 1 0^{- 7})$ $H S^{-} ⇌ H^{+} + S^{2 -}, K_{a 2} = \frac{[ H ^{+} ] [ S ^{2 -} ]}{[ H S ^{-} ]} (\sim 1 0^{- 13}, very small)$

Inhe alag kyun rakhein? Kyunki $K_{a 2}$ bahut hi minuscule hai, strongly acidic solution mein dominant dissolved sulphide species actually $H S^{-}$ hoti hai (aur mostly undissociated $H_{2} S$ ); free $S^{2 -}$ vanishingly small hoti hai. Dono steps ko ek overall $K_{a}$ mein lump karne se yeh chhup jaata hai aur tum mislead ho jaate ho ki solution mein kya actually hai.

Step 2 — $[S^{2 -}]$ ko $[H^{+}]$ ke terms mein nikalo. Dono equilibria multiply karo ( $K_{a} = K_{a 1} K_{a 2}$ ) aur solve karo: $[S^{2 -}] = \frac{K _{a 1} K _{a 2} [ H _{2} S ]}{[ H ^{+} ] ^{2}}$

Yeh step kyun? Yeh dikhata hai ki $[S^{2 -}] \propto 1/ [H^{+}]^{2}$ . Kam pH (zyada $H^{+}$ ) ⇒ tiny $[S^{2 -}]$ ; zyada pH ⇒ bada $[S^{2 -}]$ . Dhyan raho ki $[H_{2} S]$ roughly constant rehta hai (saturated, $\sim 0.1$ M) kyunki yah barely ionised hota hai.

Step 3 — precipitation condition apply karo $[M^{2 +}] [S^{2 -}] > K_{s p}$ .

Group II mein H₂S se pehle HCl kyun add karte hain? HCl se $H^{+}$ suppress karta hai $H_{2} S$ ionisation ko (common-ion effect), $[S^{2 -}]$ low rakhta hai taaki Group IV ions prematurely precipitate NA hon.

WHY Group III, Group IV se pehle?

Worked example — separations predict karna

Common mistakes (Steel-man + fix)

Recall Feynman: ek 12-saal ke bachche ko samjhao

Socho ek crowd hai invisible bachchon ki jinhein 5 buses mein sort karna hai. Tum unhe dekh nahi sakte, lekin har group ek alag "call" pe respond karta hai. Pehle tum call karo "woh saare bachche jinhe salt water se nafrat hai!" — Bus 1 (HCl) bhar jaati hai. Phir "khatte juice mein stink-gas se darne wale bachche!" — Bus 2 bhar jaati hai. Phir ek softer call, phir stink-gas phir se lekin meethe juice mein, phir ek fizzy-rock call. Sabse choosy group ko pehle call karke, har baar sirf sahi bachche board karte hain, aur koi galat bus mein nahi ghusta.

Flashcards

Group I cations ka group reagent kya hai?

Dilute HCl (Ag⁺, Pb²⁺, Hg₂²⁺ ko chlorides ke roop mein precipitate karta hai)

Group II ka group reagent kya hai?

H₂S in acidic (dil. HCl) medium → low-Ksp sulphides

Group II mein acidic medium kyun use karna zaroori hai?

Acid [S²⁻] low rakhta hai (kyunki [S²⁻]∝1/[H⁺]²), toh sirf bahut-chhote-Ksp wale sulphides precipitate hote hain, II ko IV se alag karte hue

Strongly acidic solution mein dominant dissolved sulphide species kaun si hai?

HS⁻ (aur undissociated H₂S); free S²⁻ vanishingly small hai kyunki Ka₂ ≈ 10⁻¹³

Group III ka group reagent kya hai?

NH₄Cl + NH₄OH → Fe³⁺, Al³⁺, Cr³⁺ ke hydroxides

Group III mein NH₄Cl ka role kya hai?

Common-ion effect se [OH⁻] low karta hai, Group IV hydroxides ko precipitate hone se rokta hai

Group IV ka group reagent kya hai?

H₂S in basic (NH₄OH) medium → higher-Ksp sulphides (Co,Ni,Mn,Zn)

Group V ka group reagent kya hai?

(NH₄)₂CO₃ in ammoniacal medium → Ba²⁺,Sr²⁺,Ca²⁺ ke carbonates

[S²⁻] aur pH mein kya relation hai?

[S²⁻]=Ka₁Ka₂[H₂S]/[H⁺]² ; [H⁺] kam hone pe (higher pH) badhta hai

Group II mein As, Sb, Sn ki oxidation states kya hain?

As³⁺, Sb³⁺, Sn²⁺ (apne sulphides ke roop mein precipitate hote hain)

Pb²⁺ Groups I aur II dono mein kyun aata hai?

PbCl₂ partly soluble hai, toh bacha hua Pb²⁺ Group II mein carry ho jaata hai aur PbS ke roop mein precipitate hota hai

Koi bhi precipitation shuru hone ki condition kya hai?

Ionic product Q, Ksp se exceed kare (Q > Ksp)

Group analysis ka sahi order kya hai?

I → II → III → IV → V (sabse selective reagent pehle)

Kaun se ions Groups I–V mein nahi hain?

Mg²⁺, Na⁺, K⁺, NH₄⁺ (Group VI / zero — flame & special tests)

Connections

Solubility Product Ksp — har separation ka quantitative engine
Common Ion Effect — kyun Gp II mein HCl aur Gp III mein NH₄Cl selectivity dete hain
Weak Acid Dissociation — H2S — do-step pH→[S²⁻] link (Ka₁, Ka₂)
Confirmatory Tests for Cations — grouping ke baad kya karte hain
Le Chatelier Principle — yahan use hone wale equilibrium shifts ko drive karta hai